Standards for serial buses for realizing high-speed data transfers include IEEE 1394 standard. Various technologies employing the IEEE 1394 standard have been proposed until present. One such technology is disclosed in the Japanese Patent Applications Laid-Open No. 11-212896 and 2000-253023.
The IEEE 1394 standard supports isochronous transfers and asynchronous transfers. Isochronous transfers are transfers which are time-managed in 125 μs cycles (isochronous cycles) and have a constant transfer bandwidth. As the transfer bandwidth is reliably secured for this type of transfers, isochronous transfers are suitable for transfers of video images and the like wherein interruptions are not permitted. Alternatively, asynchronous transfers are transfers wherein data transfers are reliably carried out without any time management.
FIG. 21 depicts a format of a cycle start packet which is transmitted in each isochronous cycle. The cycle start packet is made up of 32-bit data sequences (quadrate data). A destination_ID refers to an identifier for a transmission destination, while a source_ID refers to an identifier for a transmission source. A transaction label tl is a label for identifying various transactions. Also, rt indicates a retry code, tcode indicates a transaction code, pri indicates a priority order, destination_offset indicates packet type-inherent information and cycle_time_data indicates packet type-inherent quadrate data, respectively. A header_CRC indicates cyclic redundancy check (hereinafter referred to as CRC) data for checking for errors error in the 4 quadrate data items.